Former Vice President Walter Mondale and two influential environmental advocacy organizations have joined the government’s side in a U.S. Supreme Court case that could have significant implications for the St. Croix River.

The 165-mile river was put into federal protection decades ago for the enjoyment of everyone, Mondale argued in Joseph P. Murr vs. Wisconsin and St. Croix County, which weighs the extent of private property rights within riverway boundaries. Mondale was joined in the amicus brief by the St. Croix River Association and American Rivers.

“To allow unchecked private development would prioritize the preferences of shoreline property owners, who constitute a minority of river users, to the detriment of the majority of people who recreate on the St. Croix River,” wrote attorneys Hope M. Babcock and Daniel H. Lutz.

In April, four of the six children of William and Margaret Murr appealed to the high court, contending that St. Croix County wrongly blocked them from selling a vacant waterfront parcel. The Murrs said they wanted to sell the 1.25-acre lot to finance improvements to their summer home, built soon after their parents bought the property in 1960 on an adjoining lot south of Hudson, Wis.

St. Croix County argued that zoning ordinances governing the river prohibit such land use. But the Murrs alleged that the county’s denial of the sale violated the Fifth Amendment’s “takings clause,” because it amounted to taking private property for public use without just compensation.

“The regulatory environment on the St. Croix is not what this case is about,” the Murrs’ attorney, John Groen, said recently. “The regulatory environment is simply the vehicle that raised that legal issue. This isn’t a challenge to … rules that protect the river.”

But Mondale and the environmental groups say the case is all about the river’s “outstandingly remarkable values,” adding that concern over land development led to designating the river for federal protection.

During a debate in Congress in 1972, then-Sen. Mondale described the river as “probably the last remaining unpolluted, scenic river left in the nation next to a major metropolitan area.” Failing to protect it, he said, would lead the river “into ugly, desecrated sewage.”

A desire for protection began with the mid-1960s construction of a power plant between Stillwater and Bayport, “drawing widespread public calls to protect the St. Croix River from development and pollution,” the Mondale brief said.

In a twist, proponents of a new four-lane bridge used the presence of the Xcel Energy plant and its towering smokestack to persuade members of Congress to exempt the bridge from the Wild and Scenic Rivers law. Construction began in 2013 and will continue into 2017.

“I was opposed to it, but it’s there and we’ll live with it,” Mondale has said of the bridge.

The Murr case is scheduled for oral arguments in October. Groen, who represents the Pacific Legal Foundation, said a Supreme Court ruling favorable to the Murrs could benefit private property owners nationwide.

But Mondale and the environmental groups warned against giving landowners power over regulations that protect the St. Croix. “High quality … recreation can motivate an increase in private development to capture higher property values, which, in turn, will destroy the values of the river that attract people to it in the first place,” their brief said.